<!DOCTYPE html>
<html>
    <head>
        <meta charset="utf-8">
        <title>随机美女图片</title>
        <style type="text/css">
            .bg {
                width: 100%;
                height: 100%;
                position: fixed;
                z-index: -1;
                background-position: center;
                background-repeat: no-repeat;
                background-attachment: fixed;
                background-size: cover;
                animation: bg 10s infinite;
            }
            #bg1 {
                background-image: url('https://cdn.seovx.com/?mom=302&1');
            }
            #bg2 {
                animation-delay: 5s;            /* bg2延迟5秒开始。同时bg2一开始无图片，使bg1淡入时背景为白色而不是另一张图片 */
            }
            @keyframes bg{
                0%{transform: scale(1.5, 1.5);opacity: 0;}
                20%{opacity: 1;}                /* 0~2s 载入 */
                30%{transform: scale(1, 1);}            /* 0~3s图片缩放 */
                50%{opacity: 1;}                /* 5~7s 淡出 */
                70%{opacity: 0;}                /* 7~10s 图片隐藏。另一个背景在本背景淡出同时载入 */
                100%{opacity: 0;}
            }
        </style>
    </head>
    <body>
        <div id="bg2" class="bg"></div>
        <div id="bg1" class="bg"></div>
    </body>
    <script type="text/javascript">
        countImg = 1;
        countTime = 0;
        bg1Container = document.getElementById("bg1");
        bg2Container = document.getElementById("bg2");
        setInterval(function(){
            // 一个轮回为10秒（2个图片，第1张0-5s，第2张5-10s），每个背景在淡出后（第7秒）更新图片
            countTime += 1;
            if(countTime % 10 == 7){
                countImg += 1;
                bg1Container.style = `background-image: url(https://cdn.seovx.com/?mom=302&${countImg})`;
            }else if((countTime + 5) % 10 == 7){    // 第2张图片默认无图片，比第一张延迟5s开始动画，+5保证了在第2秒开始加载图片
                countImg += 1;
                bg2Container.style = `background-image: url(https://cdn.seovx.com/?mom=302&${countImg})`;
            }
        }, 1000);
    </script>
</html>